In paper making, cellulosic pulp is prepared by mechanical treatment in beaters which increases the surface area of fibers in the pulp by cutting, fibrillation and hydration. Subsequently, a dilute suspension of the pulp along with other materials (like alum, resin and fillers) is filtered on a wire screen. Water drains off through the wire screen and a wet fibrous mat retained on the wire screen is mechanically processed and dried. "Wet-end" additives are added as dispersions in water at a suitable stage to the pulp slurry prior to sheet formation.
In paper manufacture, starch derivatives may be used as wet-end additives to improve the processability (drainage and retention) and strength properties of paper. Cationic and amphoteric starches are, thus, widely used in the paper industry. Graft copolymers of starch with cationic monomers like dialklamino-alkyl(meth)acrylamide (JP 88,219,696, JP 87,104,998), dimethylaminopropyl acrylamide (JP 88,275,795) and 2-(methacryloyloxy) ethyltrimethylammonium Me sulfate (1986 Eur. Pat. Appl. EP 194,987) along with acrylamide (in most cases) have recently been reported as improving (when used as a wet-end additive) the drainage and/or retention during paper manufacture and/or dry strength properties of
In the process of paper manufacture, whilst we do not wish to be bound by theory, we believe, the "mileage" obtained by use of wet-end additives results from improving fiber to fiber or/and fiber to filler bonding by anchoring onto reactive sites on the fiber and/or filler.
Interfiber hydrogen bonds are formed as a result of a wet paper web drying in the absence of any additive. The cationic starch based wet-end additives are known to anchor through their cationic functional groups onto the anionic reactive sites on cellulosic fibers (cellulose fibers in water are negatively charged due to ionised groups and residual lignin present on the surface of the fibers). Thus, the combination of natural fiber-fiber interaction through hydrogen bonding and the interfiber and fiber-filling bonding through anchoring of cationic polymers helps to improve drainage, retention and strength properties in paper manufacture.
However, the improvement in processing and properties brought about by these additives is influenced by (i) presence of other cationic species in the paper furnish, and (ii) the reactivity of the fibers and fillers for the cationic additives. There exist chemicals like aluminium sulphate (alum) giving rise to cationic species in water which are used extensively in paper-making, their primary role being to set rosin size. Alum is also known to have an impact on retention, drainage, paper strength and in addition it effects the pH and total system ionic charges.
The amount of alum used in paper making depends on the conditions optimised by a particular paper manufacturing unit. It is based on various considerations, such as, quality of pulp, water and type of fillers, etc. Alum is known to effect the performance of cationic and/or amphoteric starch based wet-end additives. Aluminium sulphate in solution gives rise to Al.sup.+++, Al(OH).sub.2.sup.+, Al(OH).sup.++ and Al.sub.8 (OH).sub.20.sup.+4 species depending on the pH. These cationic species suppress the anionic charges on the fiber or filler and therefore reduce their reactivity to the cationic/amphoteric wet-end additives. Thus, although lower levels of alum are useful for rosin sizing and neutralising the anionic colloidal impurities in the furnish, higher levels of alum present in the furnish is detrimental to the performance of cationic and/or amphoteric wet-end additives.
A given paper making system can only tolerate a particular amount of combined cationic additives to deliver optimum performance as regards processing and properties of the finished product. The higher the concentration of alum used, the lower will be the performance of these polymers as wet-end additives and their use is restricted in such cases. Although alum as well as cationic polymers produce cationic species, their roles are different and many cationic polymers do not function fully in the presence of high amounts of alum. Thus, paper manufacturing processes are restricted to certain usage levels of alum/cationic polymers.
K. Tanaka, F. Masuda and K. Mita in a Japanese Patent JPB 55-42200 describes the synthesis of graft copolymers of starch and a carboxyl group - containing ethylenic unsaturated monomer, (sodium salt) and their use in strengthening of paper. The graft copolymers were synthesised from gelatinised starch. In more detail, 50 parts starch in gelatinised form is reacted with 50 parts of a carboxylated ethylenic monomer, with a synthetic monomer content of 10% to 1000% (by weight of starch) using hydrogen peroxide as catalyst. The product is neutralised with sodium hydroxide at the end of the reaction.
Treatment of paper pulp with 0.5% of this polymer is shown to improve the dry breaking length of paper at low alum levels of 0.5%, based on the weight of the pulp. The advantages described by the authors for the additive are (1) water solubility as compared to many starch derivatives used as paper additives, and (2) improved performance in improving some of the paper properties. The extent of property improvements described in this patent are also known to be achieved by commercially available cationic and amphoteric starch derivatives with the same levels of incorporation. However, the products exemplified can be expected to be costly owing to the fact that their synthesis involves the large amounts of synthetic monomers, to the difficult processing conditions associated with use of viscous starch dispersions, etc. and recovery of the product by energy intensive unit operations like drum drying.
There are no definite property attributes indicated in the aforementioned document for these additives vis-a-vis existing high performance wet-end additives under specific paper manufacturing conditions. Comparisons have been made with low performance dry strength improvement additives.
We have now surprisingly found an alternative process for making paper which may improve drainage and retention during processing and also improve the dry strength of paper. Process performance may improve with increasing amounts of alum up to a limit (about 10%) and further increases in the alum levels do not detrimentally affect the performance. Thus, the present process is also surprisingly effective for paper mills operating at high alum levels in addition to mills operating at moderate alum levels. This is in contrast to the case of some processes using cationic and/or amphoteric starch derivatives where performance is detrimentally affected due to presence of high amounts of alum.